This invention relates to a method and structure of controlling a sensor heater, and with more particularity to a method and structure of controlling a sensor heater using a pulse width modulation duty cycle.
Various types of sensors are heavily dependent on operating temperature, dictating the quality of the response value detected by the sensor. Included in this class of sensors are: pollution sensors, combustible gas sensors, organic solvent sensors, toxic gas sensors, VOC sensors, as well as other sensors known in the art. Therefore, manufacturers of these types of sensors generally include heater elements associated with the sensors so that they operate at elevated temperatures, sometimes in the range of greater than 400xc2x0 C. to maintain a quality of a response signal. Heaters associated with these types of sensors generally consume a constant power such that they can dissipate a constant amount of heat. If the heaters do not consume a constant power, a sensor response may be inaccurate. Various manufacturers of sensors recommend that the heaters be powered by a specific constant direct current voltage to maintain an accurate and constant power dissipation.
Sensors currently utilized in the art have problems dissipating the heat created through the operation of the sensor. Heat is generated from both the heater, as well as the power supply electronics necessary for operating the sensor. The power supply electronics convert a power supply voltage to a specific voltage needed to drive a sensor""s heater. In typical automotive applications for example, a power supply voltage can range from 9 to 16 volts, while a sensor heater voltage must be stepped down to operate at a lower voltage usually in the range of from 5 to 7 volts. However, in various commercial applications, power supply voltages can be increased in the range of up to 24 volts. Therefore, the heat that must be dissipated to maintain a constant operating temperature of a sensor in such situations is large.
It is known in the art that linear regulators may be utilized to supply a constant voltage to a device that is lower than the supply voltage by dissipating power through a linear regulator such that the energy is dispersed in the form of heat. When utilizing such a method, often a large expensive heat sink may be utilized to dispense the heat. Such expensive heat sinks occupy a significant amount of space, as well as contribute to an overall cost of a sensor.
One method of solving the above problem of dissipating heat is by pulse width modulating (PWM) the voltage which powers a sensor heater. Pulse width modulation is a method of turning a load on and off very quickly through a switching device. Typically, a full supply voltage is switched off and on at a specific duty cycle less than 100 percent. As the voltage is switched on and off, the same amount of power is delivered to the load, but less power is consumed by the switching device than by a linear regulator, as referenced above. A heater associated with a sensor must dissipate a constant amount of heat to maintain the temperature and therefore the accuracy of the sensor. The power dissipated through the heater is generally a function of a voltage passed through the heater element, as well as the current running through it. One problem typically associated with a PWM circuit is that the current running through the heater must be known. It is often difficult to measure current directly and cost-effectively in an electronic circuit. Therefore, indirect methods are used to measure a current. For example, a known pulse width modulation circuit for a sensor utilizes a low resistance value, high wattage tight tolerance resistor that is placed in series with a heater associated with the sensor. The voltage across the sense resistor is measured utilizing an analog to digital converter on a microcontroller. Another analog to digital converter measures a voltage provided to the heater. Therefore, in order to calculate a current running through the heater, the voltage is measured on each side of the sense resistor using the analog to digital converters and the difference is divided by the sense resistor""s known resistance value. While this arrangement provides a method of pulse width modulating a heater for a sensor, the control circuit is complex and requires three input/output lines of a microcontroller. Typically, one input and output is used to determine the system voltage, another is needed to provide a feedback voltage, and a third is utilized to control a switch or relay to pulse width modulate the voltage for the heater based on the previous inputs.
There is, therefore, a need in the art for a control structure for a sensor heater, as well as a method of controlling the sensor heater without the use of expensive heat sinks and large microcontrollers, as well as provides a reliable means of controlling a power dissipation of a sensor heater.
A control structure for a sensor heater including a power supply connected to a switching device for pulse width modulating a voltage from the power supply. The switching device is further connected to a resistance heater associated with a sensor. A microcontroller having a single output is connected to the switching device. A single input of a microcontroller is connected to a high side of the resistance heater. The microcontroller determines a pulse width modulation duty cycle to maintain a constant power dissipation of the resistance heater. This is done without the need to measure/calculate the current.
Also, disclosed is a method of controlling a sensor heater including the steps of (a) providing a microcontroller for adjusting a pulse width modulation duty cycle of a heater, (b) measuring a peak voltage at the high side of a resistance heater, (c) determining a pulse width modulation duty cycle according to the equation: duty cycle=Ton/Tcycle which=Vrms2/Vpeak2 where Ton is the time of the duty cycle when voltage is being transmitted to a heater and Tcycle is the total time of a duty cycle, and Vrms is a constant desired voltage and Vpeak is a voltage that is measured on a high side of the resistance heater, (d) transmitting the pulse width modulation duty cycle to the heater to maintain a constant power dissipation of the heater.